Kotlin Multiplatform Mobile

Overview

Kotlin Multiplatform Mobile (KMM) is a software development kit (SDK) from JetBrains designed to facilitate cross-platform mobile application development. Introduced in 2017, KMM allows developers to write shared business logic once in Kotlin and use it across both Android and iOS applications. The core principle behind KMM is to maximize code reuse where it makes sense—typically the non-UI parts of an application, such as data storage, networking, analytics, and complex business rules—while enabling developers to implement user interfaces natively on each platform.

For Android, KMM compiles Kotlin code to JVM bytecode, which is directly consumable by Android projects. For iOS, the Kotlin code is compiled to native binaries that can be integrated into Xcode projects, making the shared logic accessible from Swift or Objective-C code. This approach aims to provide the benefits of platform-specific UI performance and user experience, which can be critical for complex or highly interactive applications, while reducing the development effort associated with maintaining separate codebases for core functionalities.

KMM is particularly suited for development teams that already have expertise in Kotlin or are looking to adopt a single language for both their shared logic and Android-specific development. It also appeals to organizations that prioritize a native look and feel for their applications, as it does not impose a cross-platform UI framework. This distinguishes it from other cross-platform solutions like Flutter or React Native, which provide their own UI rendering engines. By maintaining native UI layers, KMM applications can adhere closely to platform design guidelines and leverage the latest UI components and features as they become available on iOS and Android, such as SwiftUI or Jetpack Compose.

The developer experience with KMM involves writing shared modules in Kotlin, which can then be consumed as a standard Gradle dependency in Android projects and as a framework in iOS projects. Tooling support is integrated into Android Studio and Xcode, providing a workflow for debugging and testing across both platforms. This allows developers to maintain tight control over the native aspects of their applications while benefiting from the efficiencies of a shared codebase for business logic.

Key features

• Shared Business Logic: Enables writing core application logic, such as data management, networking, and algorithms, once in Kotlin for use on both Android and iOS platforms. This reduces code duplication and helps ensure consistent behavior across apps.
• Native UI Integration: KMM allows developers to build user interfaces using native UI frameworks like Jetpack Compose or XML for Android and SwiftUI or UIKit for iOS. This approach supports platform-specific design patterns and performance characteristics.
• Kotlin Language Foundation: Based on the Kotlin programming language, KMM leverages Kotlin's features, including coroutines for asynchronous programming and a rich standard library, to write concise and expressive code. Developers can reference the Kotlin Standard Library API reference for available functions.
• Platform-Specific APIs: KMM provides mechanisms to access platform-specific APIs when needed, allowing developers to interact with device features or third-party SDKs that are unique to Android or iOS from within their shared Kotlin code.
• Tooling Support: Integrated development environment (IDE) support is available through plugins for Android Studio and Xcode, facilitating project setup, code editing, debugging, and deployment for KMM projects.
• Open Source: KMM is an open-source project, allowing community contributions and transparency in its development. This aligns with modern development practices and fosters collaboration, as detailed in the official Kotlin Multiplatform Mobile documentation.

Pricing

Kotlin Multiplatform Mobile is an open-source technology developed by JetBrains and the Kotlin community. There are no direct licensing costs or fees associated with using the KMM SDK. Developers can download and use the SDK, its tools, and libraries without charge.

While KMM itself is free, development efforts may incur costs related to developer salaries, infrastructure, and any third-party commercial tools or services chosen for specific project needs (e.g., CI/CD platforms, backend services like Firebase documentation, or analytical tools).

Common integrations

KMM projects can integrate with a wide range of mobile development tools and services. Due to its nature of sharing only business logic, integrations often occur at the shared module level (for common backend services) or at the native platform level (for UI-specific or platform-dependent SDKs).

• Networking Libraries: Integration with HTTP clients like Ktor or OkHttp for making network requests in the shared module. Ktor is a Kotlin-native framework that supports multiplatform development, providing clients for various platforms.
• Serialization Libraries: Kotlinx.Serialization for converting data between Kotlin objects and various formats like JSON, XML, or Protocol Buffers, crucial for data interchange with APIs.
• Database Solutions: SQLDelight for SQLite database access across platforms, providing typesafe SQL access from Kotlin. Realm Database also offers multiplatform support for persistent data storage.
• Dependency Injection Frameworks: Koin or Dagger (via its Android-specific implementations) can be used to manage dependencies within the shared module and across platform-specific components.
• Asynchronous Programming: Kotlin Coroutines are deeply integrated for managing asynchronous operations, making it easier to handle network calls, database operations, and other long-running tasks without blocking the UI.
• Firebase Services: While Firebase SDKs are primarily platform-specific (Android and iOS), KMM projects can integrate with Firebase by abstracting its APIs within the shared module and providing platform-specific implementations using the official Firebase SDKs.
• Payment Gateways: Integration with payment providers like Stripe or PayPal often involves using their respective native SDKs within the Android and iOS layers, with the payment processing logic coordinated by the shared KMM module. The Stripe mobile SDK documentation provides platform-specific guides.

Alternatives

Developers considering Kotlin Multiplatform Mobile often evaluate other frameworks that offer cross-platform capabilities:

• React Native: A JavaScript framework for building native mobile apps, allowing developers to write code once and deploy to both iOS and Android with a shared UI layer rendered using native components.
• Flutter: Google's UI toolkit for building natively compiled applications for mobile, web, and desktop from a single codebase, using Dart and its own rendering engine (Skia).
• Xamarin: A Microsoft-owned framework for building cross-platform apps with C# and .NET, which provides a layer to access native APIs and UI components.
• Native Development (Swift/Kotlin): Developing separate, entirely native applications for each platform using Swift/SwiftUI for iOS and Kotlin/Jetpack Compose for Android. This offers maximum control and performance but with increased code duplication.

Getting started

To begin with Kotlin Multiplatform Mobile, you typically start by creating a new KMM project in Android Studio, which sets up the necessary module structure for shared code and platform-specific applications. The following example demonstrates a simple shared Kotlin function that calculates the current day of the week, accessible from both Android and iOS.

// shared/src/commonMain/kotlin/com/example/shared/Platform.kt
package com.example.shared

import kotlinx.datetime.Clock
import kotlinx.datetime.LocalDateTime
import kotlinx.datetime.TimeZone
import kotlinx.datetime.todayIn

expect class Platform() {
    val name: String
}

fun greet(): String {
    return "Hello, ${Platform().name}! Today is ${currentDayOfWeek().name}."
}

// shared/src/commonMain/kotlin/com/example/shared/DateUtils.kt
package com.example.shared

import kotlinx.datetime.Clock
import kotlinx.datetime.DayOfWeek
import kotlinx.datetime.TimeZone
import kotlinx.datetime.todayIn

fun currentDayOfWeek(): DayOfWeek {
    return Clock.System.todayIn(TimeZone.currentSystemDefault()).dayOfWeek
}

// shared/src/androidMain/kotlin/com/example/shared/Platform.android.kt
package com.example.shared

class Platform : Platform() {
    override val name: String = "Android"
}

// shared/src/iosMain/kotlin/com/example/shared/Platform.ios.kt
package com.example.shared

import platform.UIKit.UIDevice

class Platform : Platform() {
    override val name: String = UIDevice.currentDevice.systemName + " " + UIDevice.currentDevice.systemVersion
}

// Example usage in Android (e.g., in an Activity)
// import com.example.shared.greet
// ...
// Log.d("KMM_TEST", greet())

// Example usage in iOS (e.g., in a ViewController)
// import shared
// ...
// let greeting = Shared.greet()
// print(greeting)

This snippet illustrates the core concepts: an expect declaration in commonMain for platform-specific implementations and actual declarations in androidMain and iosMain. The greet() function, part of the shared module, can then be called from native Android (Kotlin) and iOS (Swift) codebases, demonstrating how shared logic integrates into each platform's environment.